The oxygen levels of water discharged from hydraulic turbines used in hydroelectric installations have been known to provide an environmental problem due to the discharged water having relatively low levels of dissolved oxygen. The problem is more predominant in the summer months. During the summer months, thermal stratification of water in an upstream water reservoir creates warmer water near the surface having relatively high dissolved oxygen and colder water near the bottom of the reservoir having relatively lower dissolved oxygen. Because the water utilized by the turbines is usually taken from the bottom of the reservoir, this water presents a potentially environmental hazard when discharged downstream by the turbines.
In the building of new hydroelectric installations and the refurbishing of existing hydroelectric installations, there is an opportunity to improve the level of dissolved oxygen in the water by introducing oxygen to the water as it passes through the turbine. It is known to admit air through one or more runner blades in the turbine as disclosed in U.S. Pat. No. 5,879,130 issued Mar. 9, 1999; U.S. Pat. No. 5,896,657 issued Apr. 27, 1999; U.S. Pat. No. 5,924,842 issued Jul. 20, 1999; U.S. Pat. No. 6,155,783 issued Dec. 5, 2000; U.S. Pat. No. 6,247,893 issued Jun. 19, 2001, and U.S. Pat. No. 6,454,533 issued Sep. 24, 2002. However, each of these turbines requires introducing air through at least one of the moveable turbine runner blades. This requires complex blade manufacture and the introduction of air through a moving part.
It is also known to admit air around a Francis turbine band as disclosed in U.S. Pat. No. 5,823,740 issued Oct. 20, 1998. The turbine has a rotatably mounted runner having a band, a stationary discharge ring and a seal between the band and the discharge ring. While the band seal limits water leakage into a band mixing chamber formed between the runner band and the stationary discharge ring, the mixing chamber is substantially flooded with water during turbine operation. An oxygen containing gas is injected into the chamber through a gas admission aperture located near the seal. The mixing chamber is configured to promote mixing of the oxygen containing gas and water leaking into the chamber. The gas and water mixture is then discharged through an outlet port between the discharge ring and the runner band. However, by having the chamber substantially flooded with water, the amount of oxygen introduced into the water flowing through the turbine is limited.
Accordingly, there is a need for improving the transfer of an oxygen containing gas into the turbine water passageway from a chamber surrounding the runner band where the chamber is substantially unflooded.